1. Field of the Invention
The present invention relates, in general, to the removal of contaminants from flue gas, and, in particular, to a new and useful method to recover useful heat while removing particulates (fly ash), sulfur oxides and/or other contaminants contained in flue gases formed during the combustion of waste materials, coal and other fossil fuels, which are burned by electric power generating plants, waste-to-energy plants and other industrial processes through the use of a two-stage downflow flue gas treatment condensing heat exchanger.
2. Description of the Related Art
In the power generating field, there are several known devices and methods which relate to the integrated heat recovery and pollutant removal of particulates, sulfur oxides and/or contaminants from a hot combustion exhaust gas for complying with federal and state emissions requirements.
One device which has been used is a condensing heat exchanger, as shown in FIG. 1, which recovers both sensible and latent heat from flue gas 11 in a single unit 10. The device allows for the gas 11 to pass down through a heat exchanger 12 while water 14 passes upward in a serpentine path through the tubes of heat exchanger 12. Condensation occurs within the heat exchanger 12 as the gas temperature at the tube surface is brought below the dew point. The condensate falls as a constant rain over the tube array of heat exchanger 12 and is removed at the bottom by condensate drain 16. Gas cleaning may occur within the heat exchanger 12 as the particulates impact the tubes and gas condensation occurs.
The heat exchanger tubes and inside surfaces of the heat exchanger shell are made of corrosion resistant material or are covered with Teflon.RTM. in order to protect them from corrosion when the flue gas temperature is brought below the acid dew point. Interconnections between the heat exchanger tubes are made outside of the tube sheet and are not exposed to the corrosive flue gas stream 11.
Another device used in this area is an integrated flue gas treatment (IFGT) condensing heat exchanger 20, schematically shown in FIG. 2, which is a condensing heat exchanger designed to enhance the removal of pollutants from flue gas stream 22. It is also made of corrosion resistant material or has all of the inside surfaces covered by Teflon. There are four major sections of the IFGT 20: a first heat exchanger stage 24, an interstage transition region 26, a second heat exchanger stage 28, and a mist eliminator 30. The major differences between the integrated flue gas treatment design of FIG. 2 and the conventional condensing heat exchanger design of FIG. 1 are:
1. the integrated flue gas treatment design uses two heat exchanger stages 24 and 28 instead of one heat exchanger 12 (FIG. 1); PA1 2. the interstage or transition region 26, located between heat exchanger stages 24 and 28, is used to direct the gas 22 to the second heat exchanger stage 28, and acts as a collection tank and allows for treatment of the gas 22 between the stages 24 and 28; PA1 3. the gas flow in the second heat exchanger stage 28 is upward, rather than downward; PA1 4. gas outlet 29 of the second heat exchanger stage is equipped with an alkali reagent spray system, generally designated 40, comprising reagent source 42 with a pump 44 for pumping reagent 42, recirculated condensate, and spent reagent to sprayers 46 and 48; and PA1 5. the mist eliminator 30 is used to separate the water formed by condensation and sprays from the flue gas.
Most of the sensible heat is removed from the gas 22 in the first heat exchanger stage 24 of the IFGT 20. The transition region 26 can be equipped with a water or alkali spray system 48. The system 20 saturates the flue gas 22 with moisture before it enters the second heat exchanger stage 28 and also assists in removing sulfur pollutants from the gas 22.
The transition piece 26 is made of or lined with corrosion resistant fiberglass-reinforced plastic or other corrosion resistant material. Additionally, the second heat exchanger stage 28 is operated in the condensing mode, removing latent heat from the gas 22 along with pollutants. Also, the top of the second heat exchanger stage 28 is equipped with an alkali solution or slurry spray device 46. The gas 22 in this stage 28 is flowing upward while the droplets in the gas 22 fall downward. This counter-current gas/droplet flow provides a scrubbing mechanism that enhances particulate and pollutant capture. The condensed gases, particulates, and reacted alkali solution are collected at the bottom of the transition section 26. The flue gas outlet 29 of the IFGT 20 is equipped with the mist eliminator 30 to reduce the chance of moisture carryover.